The present invention relates to a method and an apparatus for removing nitrogen oxides (NOx) in exhaust gas by using a solid reducing agent, bringing nitrogen oxides into contact with a catalyst, and reducing the nitrogen oxides at a high efficiency.
Concerning a method for removing nitrogen oxides from exhaust gas discharged from a gas turbine, a gas engine, a diesel engine, a heating furnace, various boilers and the like, the selective catalytic reduction method using ammonia as a reducing agent has conventionally been applied in the de-NOx process of various stationary NOx generation sources because of the possibility to selectively remove nitrogen oxides at a high efficiency without being affected by the oxygen concentration in the exhaust gas. Ammonia is supplied in the form of liquid ammonia or aqueous ammonia.
Cogeneration systems using gas turbines, gas engines and diesel engines as driving sources have recently been rapidly increasing in number in coastal and city areas in view of global environmental protection and economical considerations, and along with this, installation of a de-NOx process in buildings and residential areas is required. In this case, the use of liquid ammonia and aqueous ammonia as a reducing agent is restricted under the regulations such as the Poisonous and Deleterious Substances Control Law, the High-Pressure Gas Control Law and the Fire Prevention Law (all of which are Japanese laws), and in addition, a special care must be taken for handling, transportation and storage thereof to prevent leakage of odors.
To solve the above problems, it is necessary to use an easy-to-handle and highly stable reducing agent in place of ammonia. Japanese Official Patent Provisional Publication No. 2-194817 discloses a method of removing NOx using a solid reducing agent such as urea, melamine, and cyanuric acid in the solid or liquid state as the easy-to-handle and highly stable reducing agent.
The aqueous solution of the solid reducing agent permits easy handling such as transportation and storage, but since the weight of water becomes an excessive load, the solid reducing agent is often handled in the form of an aqueous solution having a high concentration near the saturated concentration which gives favorable cost merits.
The aqueous solution of solid reducing agent is sprayed from a nozzle provided in the duct into exhaust gas flowing through the duct and is mixed with the exhaust gas.